This invention relates to the measurement of the sum of a linear dimension of particles in a collection of particles by the use of the forward scattered light produced when a light beam is directed at the particle collection. More specifically, this invention relates to a measurement of the sum of the radii of the particles when they are spherical. There is specifically set forth the shape of a mask which can be used in the Fraunhofer plane for filtering the forward scattered light from a collection of particles so that a measurement of the light flux transmitted by that filter is directly related to the sum of the radii of the particles in the collection. The particular design for the mask disclosed is useful in optical systems such as that shown in U.S. Pat. No. 3,873,206 entitled "Method for Determining a Specific Characteristic of Fluid Suspended Particles", issued to William Leslie Wilcock on Mar. 25, 1975. That particular patent discloses a filter mask shaped to provide a third power response, that is a measure of the total volume of the particles in the collection being examined. A substitution of the mask design set forth in this application in the system of the Wilcock patent would provide an output which is proportional to the sum of the radii of all of the particles in the collection.